x Turn on thread page Beta
 You are Here: Home >< Maths

# C4 Implicit Equation watch

1. Hi,

So I have tried the following question a number of times and continue to fail to get the answer, I think this is because of an incorrect rewriting of (xy).

So the question is:

Find an expression in terms of x and y for dy/dx, given that:
(xy) + x + y^2 = 0

Firstly could someone tell me the correct expansion of (xy) as I am writing it as (xy)^1/2 and getting confused from there. So if someone could write this as x^ay^b instead I feel that that would help a lot (:

P.S Wasn't sure how to write it so x^a means x to the power of a...
2. (Original post by BryonyG98)
Hi,

So I have tried the following question a number of times and continue to fail to get the answer, I think this is because of an incorrect rewriting of (xy).

So the question is:

Find an expression in terms of x and y for dy/dx, given that:
(xy) + x + y^2 = 0

Firstly could someone tell me the correct expansion of (xy) as I am writing it as (xy)^1/2 and getting confused from there. So if someone could write this as x^ay^b instead I feel that that would help a lot (:

P.S Wasn't sure how to write it so x^a means x to the power of a...
There's nothing really subtle going on here:

as with normal products of powers
3. (Original post by davros)
There's nothing really subtle going on here:

as with normal products of powers

Unfortunately, that is what I thought which means something else has gone wrong, time for attempt number 3!
4. (Original post by BryonyG98)
Unfortunately, that is what I thought which means something else has gone wrong, time for attempt number 3!
If you're still stuck, post your working.

I'll be offline soon but I'm sure someone else will be able to advise
5. What would x^1/2 * y^-1/2 equal... is isnt just -(xy) ​is it?
6. Is it y / x?
7. (Original post by BryonyG98)
What would x^1/2 * y^-1/2 equal... is isnt just -(xy) ​is it?

I'm going out shortly, but it might help if you posted your answer and the answer you're supposed to get.

Bear in mind that there may be several different equivalent answers because you can rationalize things or substitute from the original equation you're given to express in terms of other things!
8. (Original post by davros)

I'm going out shortly, but it might help if you posted your answer and the answer you're supposed to get.

Bear in mind that there may be several different equivalent answers because you can rationalize things or substitute from the original equation you're given to express in terms of other things!
Thank you very much, I will have one more try and then post my answer and the answer from the book (:
9. Okay so for my answer I get:

-1-y
x
2y+x
y

And the answer in the back of the book is:

2(xy) + y
4y​(xy) + x
10. Replace XY with X^(1/2)Y^(1/2) and differentiate giving you...
1/2 X^(-1/2) Y^(1/2) + 1/2 X^(1/2) Y^(-1/2) DY/DX + 1 + 2Y DY/DX = 0
Factories by the DY/DX and move any other terms other to the other side
DY/DX (1/2 X^(1/2) Y^(-1/2) + 2Y) = -1 - 1/2 X^(-1/2) Y^(1/2)
Divide by the terms on the left hand side to get DY/DX on its own and then simplify the equation to remove the fractions, although when im done i get the same answer in the book but with negatives on the top like...
-2(xy) - y
4y​(xy) + x

TSR Support Team

We have a brilliant team of more than 60 Support Team members looking after discussions on The Student Room, helping to make it a fun, safe and useful place to hang out.

This forum is supported by:
Updated: January 28, 2015
Today on TSR

### Any tips for freshers...

who are introverts?

### More snow?!

Discussions on TSR

• Latest
Poll
Useful resources

### Maths Forum posting guidelines

Not sure where to post? Read the updated guidelines here

### How to use LaTex

Writing equations the easy way

### Study habits of A* students

Top tips from students who have already aced their exams

## Groups associated with this forum:

View associated groups
Discussions on TSR

• Latest

The Student Room, Get Revising and Marked by Teachers are trading names of The Student Room Group Ltd.

Register Number: 04666380 (England and Wales), VAT No. 806 8067 22 Registered Office: International House, Queens Road, Brighton, BN1 3XE